At present, a great number of paper containers are used for beverages, such as vegetable juice, milk, yogurt, Japanese tea and coffee, alcoholic drinks, such as sake and Japanese spirits, liquid seasonings, such as sauce for noodles and soy sauce, cups, such as for instant noodles and for ice cream, and the like. Laminates for paper containers are frequently used for the paper containers, wherein a polyolefin resin layer, particularly, a polyethylene resin layer is laminated as the inner surface layer for imparting water resistance and for facilitating fabrication into the paper container by heat sealing. However, polyethylene resin has a problem of adsorbing flavor components represented by d-limonene to lose flavor-keeping, and therefore, they could not be used for beverages to which flavor-keeping is important.
Incidentally, PET resin is a most common resin not to adsorb flavor components which is inexpensive, and it is used for fruit juice drinks as PET bottles. However, since PET resin is a crystalline resin, it could not be used as it is due to its poor low temperature heat sealability which is necessary for fabricating paper containers.
Thereupon, several techniques have been proposed to blend polyethylene with polyester made of terephthalic acid and isophthalic acid and ethylene glycol (see Patent Document 1), to use non-crystallin-low crystalline modified polyethylene terephthalate having a glass transition point of 40° C. or more (see Patent Document 2), or to use a polyester consisting of 97-50 parts by weight of copolyester, wherein 50 mol. % or more of dicarboxylic acid component is terephthalic acid and diol component is composed of 40-90 mol. % of ethylene glycol and 10-60 mol. % of ethylene oxide adduct of a bisphenol, and 3-50 parts by weight of epoxy group-containing ethylene-based copolymer (see Patent Document 3).
However, the technique proposed in the above Patent Document 1 has a problem to lack a low temperature heat sealability (around 130° C.) comparable to polyethylene resin, and moreover, it is possible to adsorb flavors according to the blended volume of the polyethylene resin due to blending it. The techniques proposed in the above Patent Documents 2 and 3 have a problem that the resin employed is not a commonly produced PET resin, but is needed to be newly synthesized, and therefore, increases cost.
Then, the applicant has proposed laminated material for paper containers and a paper container using it which have solved the above problems (see Patent Document 4). The laminated material for paper containers and the paper container using it are manufactured by adding a chain extender composed of styrene-methyl (metha) acrylate-glycidyl methacrylate to PET resin having an intrinsic viscosity of 0.55-0.7 dl/g, charging the mixture to an extruder having two or more vent holes, sucking to degas with a high vacuum of −750 mmHg or more from the vent holes under heating to melt conditions of the PET resin, and thereby extrusion properties are improved, then extruding to laminate a PET resin layer on a paper layer, and immediately after laminating, cooling rapidly the face of the PET resin layer by a cooling roll, and the resulting PET resin layer has a crystal portion of less than 15% and a non-crystal portion of 85% or more represented by the following formula:
                                          crystal            ⁢                                                  ⁢            portion            ⁢                                                  ⁢                          (              %              )                                =                                                                                                                                        heat                        ⁢                                                                                                  ⁢                        quantity                        ⁢                                                                                                  ⁢                        of                                                                                                                                                fusion                        ⁢                                                                                                  ⁢                        per                        ⁢                                                                                                  ⁢                        mole                                                                                            -                                                                                                    heat                        ⁢                                                                                                  ⁢                        quantity                        ⁢                                                                                                  ⁢                        of                        ⁢                                                                                                  ⁢                        cold                                                                                                                                                crystallization                        ⁢                                                                                                  ⁢                        per                        ⁢                                                                                                  ⁢                        mole                                                                                                                                                                                    heat                      ⁢                                                                                          ⁢                      quantity                      ⁢                                                                                          ⁢                      of                      ⁢                                                                                          ⁢                      fusion                      ⁢                                                                                          ⁢                      per                      ⁢                                                                                          ⁢                      mole                                                                                                                                  of                      ⁢                                                                                          ⁢                      perfect                      ⁢                                                                                          ⁢                      crystal                      ⁢                                                                                          ⁢                      of                      ⁢                                                                                          ⁢                      PET                      ⁢                                                                                          ⁢                                              (                                                  26.9                          ⁢                                                                                                          ⁢                          KJ                                                )                                                                                                                  ×            100                          ⁢                                  ⁢                                  ⁢                              non            ⁢                          -                        ⁢            crystal            ⁢                                                  ⁢            portion            ⁢                                                  ⁢                          (              %              )                                =                      100            ⁢                          -                        ⁢            crystal            ⁢                                                  ⁢            portion                                              [                  Mathematical          ⁢                                          ⁢          1                ]            